Extrusion coating of substrates for producing plastic-coated products generally involves the application of a liquid curtain of thermoplastic resin into the nip of a pair of pressure rolls which press a thin, relatively uniform, thermoplastic coating against the substrate. See U.S. Pat Nos. 3,340,124, 3,356,556 and Japan Sho-47-46264. As these disclosures suggest, extrusion coating of paper is often concerned with providing or achieving selvedge. "Selvedge" is the term used for the uncoated portion along the edge of selected papers. The uncoated edge is valuable in the making of tubes and other configurations, since it can be easily glued. Another popular use is the fastening of one sheet to another in the manufacture of plastic-coated paper bags.
Those skilled in the art of paper manufacturing have demonstrated a desire for a manufacturing method capable of producing a selvedge edge which has an even coating of plastic at its delineation. It is known that the edge of an extruded sheet of a plastic laminant is sometimes thickened into a bead, resulting in a defect in the coated paper. One way of avoiding this beaded or uneven surface is to spray liquid onto areas of the base sheet substrate to prevent adhesion of the irregular plastic edge. See Japan Sho-47-46264. This reference further teaches the use of a knife for cutting unadhered plastic in a machine direction.
Others have achieved selvedge by splitting the molten curtain with an electrical hotwire and thereafter preventing adhesion by using an interlayer belt disposed between the plastic and substrate. See U.S. Pat. Nos. 3,340,124 and 3,356,556. These references further teach the use of a chill roll disposed to contact the thermoplastic curtain upon extrusion. These chill rolls are employed primarily to provide the necessary release of the plastic film after extrusion. See, e.g., col. 3 of U.S. Pat. No. 3,356,556.
Although these prior art methods and apparatus have been useful in applying layers of thermoplastic material to paper substrates, there still remains a need for a facilitated method for controlling the level of adhesion between a plastic coating and a base sheet substrate for providing selvedge and for minimizing imperfections in the plastic coating.